Carb blow back

[Ji Gantor]

I have a CZ400 1973 that I removed the spark plugs and Carburettor. I then sprayed some WD40 into the cylinder to lubricate the rings and placed a rag in the intake manifold to prevent dirt entry. While in this state I depressed the kickstarter lever to crank the engine. On the third crank the rag was blown out of the intake manifold. I then placed my hand over the intake manifold and cranked the engine. On the second stroke of the piston I was getting blow back.

Is this normal for two stroke engines.

Ji

[Lozza]

Yes, and the other would have been suction.

[Ji Gantor]

Thanks Lozza,
I have been reading how a two stroke engine works since I posted this question.
When the piston is on the way down the crank shaft area is in compression. The piston seals the exhaust and transfer ports so the only exit is through the inlet port to the carb.

Ji

[Lozza]

That's only true in a 'static' sense in a dynamic sense (engine running) it a fair bit different.

[Ji Gantor]

While the engine is running I know that the window of opportunity for this to happen is brief . It must be more noticeable at low revs like when I actuated the kick starter.

Ji

[David Lahey]

That effect is also why a carby jetted for a reed valve motor will usually require large jetting changes to work well on a similar sized non-reed motor.

[Lozza]

True Ji hmmm not convinced of that feetup as the fuel is metered well before the reed or the inlet port, only the amount of fuel will be metered in line with the signal from the engine.

[Ji Gantor]

Does the reed valve close off enough to prevent any negitive pressure from getting back to the carb, thus smoothing out the flow of mixture.

Ji

[Lozza]

Not really, the reeds open only on the upstroke of the piston and rapidly drop just after tdc, at this point on a piston port the inlet is still open but there is no negative pressure in the cases it has long been equalised.There is no flow 'from' negative pressure only flow to negative pressure.The pressure that blew the rag out was the pressure from piston decent.

[Ji Gantor]

Thanks mate.

Ji

[David Lahey]

Lozza I agree with what you are saying about a reed valve motor. My point is that the carby on a non-reed motor sees a different set of conditions to one on a reed valve motor.
There is two-way air flow through a piston port carby and one-way air flow through a reed induction carby.
The total flow through the venturi on a piston port carby (at the same RPM and throttle opening) is greater than the total flow through the carby on a reed motor of the same capacity because the total flow is the sum of the flows in both directions. The overall total amount of air going into the motor is the same, but the venturi sees a higher total flow. The carby adds fuel to the air whichever way it goes through the venturi.

[Ji Gantor]

Yep the petrol and air go in and the smelly gas come out.


I think Ive got it

Ji